authfs: create a chunked reader with fs-verity verification

The new chunked reader uses a Merkle tree to verify each chunk read of
the corresponding backing file. The reader also accepts an
autheneticator for signature verification, though it is currently a fake
implementation due to the lack of PKCS#7 signature support in BoringSSL
(b/170494765).

Test: atest authfs_host_test_src_lib
Bug: 171310075

Change-Id: Ibf4151ab2a93f7515ad8c9c0462df6c21c10d767
diff --git a/authfs/src/lib.rs b/authfs/src/lib.rs
index 26de157..05070d6 100644
--- a/authfs/src/lib.rs
+++ b/authfs/src/lib.rs
@@ -22,6 +22,7 @@
 //!
 //! The implementation is not finished.
 
+mod auth;
 mod crypto;
 mod fsverity;
 mod reader;